Asset Manager replacement

Well, never tried it before but I think you can achieve it just by replacing the current command

editor.Commands.add('open-assets', {
     run(editor) {
          ...
    }
})

This is how the command is called https://github.com/artf/grapesjs/blob/e921165a8d681b5d1056c816f88dd7719d4f61e0/src/dom_components/view/ComponentImageView.js#L68

the target is actually all what you need. When you select the element from a custom manager you just update the target with target.set('src', url)

UPDATE: new is API available for AssetManager customization https://grapesjs.com/docs/modules/Assets.html#customization